symmetric amplifier

[sonnya]

Originally posted by dimitri
It is bad idea to shunt Zener (LED) by a capacitor. The capacitor will not filter unwanted noise because of the low dynamic impedance of the Zener (LED). For reducing hum you can split R7(R107) as 5.1k + 5.1k, and connect C4 (C104) (better 100u) between supply rail and the common point of 5.1k resistors. If you aware about hf LED noise, you can use R-C filter, i.e. put an extra resistor between D1 and base of Q5 and reconnect C3 between supply rail and base of Q5.

I do not agree totally. Its okay to remove C2 and C102 but keep C3 and C103.
Why?
To bypass the LED's at high frequency so you not only have a low impedance at LF but also at HF.
Then again split the circuit as Dimitri says but reduce the R in the RC filter to 10 - 20 ohm to prevent making a resonance circuit with C3 and the LED.

-------

Be carefull with large size film caps like C51. Larger values more series induction.

-------

Add a small base resistor (1 - 10Ohm) on Q9 and Q109 to prevent oscillation in the output devices.

The i can only say.... Nice circuit!... go build it.

Sonny

[dkemppai]

Originally posted by Rambi

Over the last few months I've built several prototypes of power amplifers. I started with OpAmp driven circuits

Hi,

I'm currently in the process of building a similar amplifier. Just curious, do you have any gain/phase plots or scope pictures of sine/triangle/square waveforms at test frequencies? Say 1K,10K,20K, 50K, and 100K @8Ohms load or so. Also have you tested stability with any comples loads?

I'd be really curious to see how your amp responds.

Also, Am I mistaken, or is the LF411 an integrator to compensate for the DC-Bias? I'd never seen that done, but I've been thinking about trying it (Soon).

-Dan

[Rambi]

Originally posted by dkemppai


Hi,

I'm currently in the process of building a similar amplifier. Just curious, do you have any gain/phase plots or scope pictures of sine/triangle/square waveforms at test frequencies? Say 1K,10K,20K, 50K, and 100K @8Ohms load or so. Also have you tested stability with any comples loads?

I'd be really curious to see how your amp responds.

Umm, no sorry. I do have a scope, but no digital camera.
(And at the moment the design is still changing rapidly enough not to use an ordinary camera, certainly with the excellent feedback and fresh ideas from this forum, thanks all! )
Good idea though, I'll see if I can beg, steal, or borrow a digital camera for the final design (which should be around the time pigs fly, there's always another tweak to test...)
The only complex loads it has seen are various el-cheapo loudspeakers each with variyng qualities of cables (don't have a power resistor).

Out of curiosity: is your amplifier project already in a stage where you have any schematics (that you'd like to share)?

Also, Am I mistaken, or is the LF411 an integrator to compensate for the DC-Bias? I'd never seen that done, but I've been thinking about trying it (Soon).

-Dan

Yep, it is. One of the earlier design I tried to build was the Elektor HexFet power amp, which doesn't use such a DC compensation circuit. It did work, but the DC offset was too unstable to my liking (nice to see the effect of breathing on the LTPs though ).

This circuit seems to do the job quite nicely and as a bonus it eliminates an electrolytic in the feedback path. Downside is that it needs a capacitor at the input, but that doesn't have to be as big (or an electrolytic). Come to think of it: It should be possible to connect it to the other side of the LTPs with some tweaking.
It does need a few seconds to stabilize at startup (depending on component values & matching, latest one starts from appr. -1 V and needs a few seconds to get close to zero).

Remco

[dkemppai]

b]Good idea though, I'll see if I can beg, steal, or borrow a digital camera for the final design (which should be around the time pigs fly, there's always another tweak to test...)
The only complex loads it has seen are various el-cheapo loudspeakers each with variyng qualities of cables (don't have a power resistor).[/b]

Tell me about the tweaks... ...I've got an amp that's almost done, 6 years it's bee almost done! I can get some images from my scope... ...if interested let me know.

My new design is mid stream, so no schematics yet. (Soon, I hope). Right not I'm focusing on phase and linearity while trying to keep the thing stable. Phase shift is around -3.5 degrees at 20Khz. (-15 deg at 100Khz) I was shooting for less than 1 degree at 20k, but that's getting difficult. Even now it's just slightly under damped.

Response at small signals is from DC to around 300Khz, dropping to 100Khz at 40W (RMS) output. 70W at 50Khz, and so on. (I consider the limit of maximum power at the point where I can start to see distortion on the scope) It'll push a few hundred watts at 20Khz, but only for short bursts (small load/heatsinks)

I really need a %THD meter, and way to measure noise levels! (Any good recomendations?)

As for the Integrator, been tossing the idea around. Just trying to think of a few places to integrate it into circuit, where it'll be unobtrusive. I may 'borrow' some of your circuit for that.

-Dan

[halojoy]

traderbam - dear friend

I like you more than most of these "yes-sayers"
you make me think
you keep me alert
traderbam

In other words
I like you very much

So much that one day I might build me a traderbam amplifier

remember?
I do

halojoy
of sweden and jesus

---------------------------------------------------------
*this is a transcription of a post of mine
in thread "greening a classic"
where traderbam has made a lot of nice thinking

[carlmart]

Remco,


Can you e-mail me?

I would like to know some more about those other designs you built and your findings.



Carlos

[Rambi]

Originally posted by dkemppai
Tell me about the tweaks... ...I've got an amp that's almost done, 6 years it's bee almost done! I can get some images from my scope... ...if interested let me know.

I'm always interested!

My new design is mid stream, so no schematics yet. (Soon, I hope). Right not I'm focusing on phase and linearity while trying to keep the thing stable. Phase shift is around -3.5 degrees at 20Khz. (-15 deg at 100Khz) I was shooting for less than 1 degree at 20k, but that's getting difficult. Even now it's just slightly under damped.

I only have simulations for phase shift etc. With component values as shown it's about 10 degrees at 20kHz.
(IIRC that is. I'm in the middle of installing my new computer. Once I have everything back on-line I'll see if I can post an updated schematic with some of the suggestions integrated).

As for the Integrator, been tossing the idea around. Just trying to think of a few places to integrate it into circuit, where it'll be unobtrusive. I may 'borrow' some of your circuit for that.

It's not mine; as I said in my first post most of it is bits and pieces from various designs (mostly Elektor stuff). The Crescendo Millenium Edition uses an integrator like this. After some searching I found that the 'Power Amp' (also Elektor) uses a variation with an integrator connected to the 'other' sides of the LTPs.

Remco

[halojoy]

It is yours.

We all have learnt from other & eachother.
That's human culture.

It's not mine; as I said in my first post most of it is bits and pieces from various designs (mostly Elektor stuff). The Crescendo Millenium Edition uses an integrator like this. After some searching I found that the 'Power Amp' (also Elektor) uses a variation with an integrator connected to the 'other' sides of the LTPs.

I have studied every single Elektor amplifier in its details.
I have almost complete collection from 1985 of Swedish elektor.
A good way of learning Electronics - not only for Audio.

halojoy, traderbam, Nelson Pass, peranders or AKSA
we all use what was brought to us by culture.

But then we add a touch of ourselves.
Our own contribution to the sum of human culture.
And so do you, my friend Rambi

Enjoy yourself - You are great!

/halojoy the great

[dkemppai]

Originally posted by Rambi
I'm always interested!

Cool, here are a few images. (I hope you can find that camera to snap a few to post also )

The measurements are pretty self explanatory, as to frequency, phase, and gain (can be calculated from peak to peak) Blue trace is output signal into 8 ohm 100W non inductive resistor. Yellow trace is input into amplifier.

I was going to grab a few more, but I left it run at high power a little too long... ...POP! SNAP! followed by orange flame from the fets, and lots of magic smoke. So, until I rebuild (In about an hour) you'll have to settle for the 100Khz and single 10Khz traces...

Also, the amp was running at +- 50 Volt rails, +- 70 volts seems to improve linearity at high frequencies.

-Dan

(EDIT Typos!)

[dkemppai]

Here's some sine waves, 100Khz.

Notice the distortion starting to appear on the rising edge. This is apparently due to the limited rising slew rate of the amplifier.

-Dan